Recently, there has been an increasing interest in energy storage technology. As the application fields of energy storage technologies have been extended to cellular phones, camcorders, notebook computers, PCs and electric cars, efforts have been increasingly been made towards the research and development of electrochemical devices. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, extensive research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy. However, such lithium ion batteries suffer from safety problems, such as fire and explosion, when encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict potential dangers (such as fire and smoke emission) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause electrical short between a cathode and an anode.
In order to solve the above safety problems of electrochemical devices, there has been proposed a separator having a porous coating layer which is formed by coating a mixture of inorganic particles and a binder polymer on at least one surface of a porous substrate. In the separator, the inorganic particles, which are present in the porous coating layer formed on the porous substrate, act as a spacer capable of maintaining the physical form of the porous coating layer, thereby preventing the porous substrate from thermal shrinkage even if electrochemical devices overheat. Also, an interstitial volume is present between the inorganic particles to form a fine pore.
Such a separator is required to adhere with an electrode in a stacking and folding process, and therefore, it is favorable for a layer for adhesion with an electrode to be significantly exposed on the porous substrate layer of the separator for good adhesion with the electrode. However, in the formation of such a layer for adhesion with an electrode, when a binder polymer is used an excessive amount, the performance of electrochemical devices may be deteriorated.